60 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



3. Put it into the bottle containing the dilute acid. After three 

 or four minutes withdraw it and again wash. If much pink 

 colour comes back, re-insert it in the acid for a short time, and 

 again wash. The process must be repeated until the film only 

 shows a slight pink tinge. 



4. Now apply the methylene blue for half a minute or so. 



5. Wash, dry with blotting-paper, and then by gentle heat. 

 Apply a drop of balsam, and cover. 



Another Method. — Filter into a test-tube sufficient carbol fuchsin 

 to flood the film, boil it over the Bunsen or spirit-lamp, and pour 

 it on to the film whilst still boiling. Let it act until it is cool, 

 when the bacilli will be found to be stained, and the process of 

 decolorization may be proceeded with. 



The times which are given above may be considerably shortened 

 in practice, but I do not advise this until considerable skill is 

 acquired. Bacilli are much more easy to recognize if they are 

 deeply stained ; this is the reason for the prolonged staining, 

 which may appear unnecessary to some. The prolonged de- 

 colorization is an advantage, since it insures that the tubercle 

 bacilli shall be the only things left stained red ; if you leave the 

 preparation in the acid for a short time, you are more likely to get 

 crystals of carbol fuchsin, stain retained in deep scratches of the 

 glass, etc., all of which a beginner may easily mistake for bacilli. 

 The counterstaining with methylene blue may be shortened or 

 omitted altogether, though this is not advisable, as it is then more 

 difficult to focus the film. 



Recognition of the Tubercle Bacillus. 



The tubercle bacillus is about half as long as a red blood- 

 corpuscle is wide, or rather longer, and is very slender. It is 

 straight or slightly curved, and is variable both in shape and in 

 size (Plate II., Fig. 2). 



We recognize it by means of a staining reaction. Tubercle 

 bacilli contain a considerable amount of fat, and this prevents 

 them from staining readily with ordinary stains. In the process 

 described above we used fuchsin, which is a very powerful stain 

 and added a mordant (carbolic acid), which increases its penetrative 

 properties. Even with this staining is very slow, so that we 

 heated the specimen. 



The fat which prevents the bacilli from staining also prevents 



